[June 12, 2009]YAKIMA, Wash. (AP) -- If workers cleaning up the nation's most contaminated nuclear site didn't have enough to worry about, now they've got to deal with radioactive wasp nests.

Mud dauber wasps built the nests, which have been largely abandoned by their flighty owners, in holes at south-central Washington's Hanford nuclear reservation in 2003.

That's when workers finished covering cleaned-up waste sites with fresh topsoil, native plants and straw to help the plants grow
-- inadvertently creating perfect ground cover for the insects to build their nests. Nearby cleanup work also provided a steady supply of mud, which the wasps used as building material.

Today, the nests, which could number in the thousands, are "fairly highly contaminated" with radioactive isotopes, such as cesium and cobalt, but don't pose a significant threat to workers digging them up.

"You don't know what you're going to run into, and this is probably one of the more unusual situations," said Todd Nelson, spokesman for Washington Closure Hanford, the contractor hired to clean up the area under the oversight of the U.S. Department of Energy.

The wasps largely built their nests in a 75-acre area around H reactor, pulling the mud from the bottom of a storage basin that once held irradiated nuclear fuel.

As for the wasps themselves, they're largely long gone -- the insects don't reuse their nests when they colonize each spring.

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb.

The site produced plutonium for the first atomic blast and for the bomb that was dropped on Nagasaki, Japan, at the end of World War II, and plutonium production continued through the Cold War.

The work left a mess of radioactive and hazardous waste to be cleaned up next to the region's largest waterway, the Columbia River. The effort is expected to last decades and cost more than $50 billion.

Workers started using excavators three weeks ago to dig up the wasp nest-infected area, including vegetation that had already been replanted. Because they are in enclosed cabs on the excavators, no protective clothing is required.

The material is then placed in a container and taken to the onsite landfill for slightly radioactive wastes, said Dave Martin, the company's radiological engineer.

Workers will eventually replant vegetation in the area, at a cost of about $25,000.